Multichip modules are increasingly being used to control operations of machines and systems. For a variety of manufacturing considerations, however, the various devices are not always installed into the multichip modules at the same time. Wire bond applications are often used though not always appropriate for certain applications. For example, a wire bond solution may not be appropriate in situations where footprint requirements necessitate efficient integrated circuit (IC) real estate usage.
An alternative approach is to use solder alloys and pastes. Using solder in either form is beneficial for bonding one die to a board, such as a ceramic or substrate board or metal lead frame, in certain circumstances. Typically, the solder alloy or paste will flow when the device and board are placed in a reflow oven or furnace and will then cool to bond the die to the board and, if desired, to create an electrical connection between the device and board. For the case of a single die, this approach is beneficial in that the die may be removed and replaced if faulty with a subsequent reflow in a reflow oven or furnace.
One issue with using a solder alloy or paste, however, is that the solder will reflow and the bond between the original die and the board will fail when the board and a new die are subsequently placed in the reflow oven to bond the new die to the board. When the bond fails, the original die may even slide off of the board or metal lead frame. One approach to solve this problem has been to use different metal alloys for the original or first die so that the metal alloys bonding the die have a higher melting temperature than what is needed for re-flowing the solder alloys of the second or new die. Two metals that have been used with this approach include lead and gold. Lead, however, is known to cause health issues and is not always desirable. Gold, on the other hand, is expensive and drives up product cost. Accordingly, solder is often used despite its limitations.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.